Field of the Invention
The present invention relates to an automatic masking device employed in a composer, composing camera and the like used for multi-duplication during the process of reproduction. More specifically, the present invention relates to an automatic masking device enabling precise exposure of the area defined by masks.
Description of the Related Art
An automatic masking device of interest to the present invention is disclosed in Japanese Utility Model Laying-Open No. 60-140960. FIG. 10 is a plan view showing the automatic masking device disclosed in this document and FIGS. 11A and 11B are cross sections of the portion shown by the lines XIA--XIA and XIB--XIB of FIG. 10. Referring to FIGS. 10, 11A and 11B, the conventional automatic masking device comprises 4 masking units 110 respectively arranged at respective end surfaces of a rectangular transparent plate 105. Each of the masking units 110 comprises a roll shaft 111 arranged in parallel to each of the end surfaces of the transparent plate 105, two pairs of flexible non-transparent masking sheets 112a and 112b arranged in intersecting directions, a moving block 124 for moving tip end portions of the masking sheets 112a and 112b to cover the transparent plate 105, a driving screw 123 for guiding the moving block 124 and the reversible motor 124 for driving the driving screw 123. The masking sheets 112a and 112b are wound around the roll shaft 111 forming winding rollers 111a and 111b. The sheets are large enough to cover from one end to the other end of the transparent plate 105 when they are yielded. Leading edges 113a and 113b for defining end portions of the masking sheets 112a and 112b are provided at end portions of the respective masking sheets 112a and 112b. A mask opening M.sub.Y in the front and rear directions is defined by 114b facing a pair of reading edges 113b in the front and rear directions, while portions facing a pair of leading edges 113a in the left and right directions define the mask opening M.sub.X in the left and right directions. The leading edges 113a and 113b are made of thick and rigid plate material so that the masking sheets 112a and 112b can be yielded against the rolling-back force of the roll shaft 111.
Generally, a masking device in a composer, for example, a horizontal composer comprises two pairs of masking sheets 112a and 112b arranged above the transparent plate 105. Plate making is carried out in the following manner by this device. An original film S is placed at a prescribed position on the transparent plate 105, as shown by a chain dotted line in FIG. 10. The two pairs of masking sheets 112a and 112b are arranged on the transparent plate 105 to cover four end surfaces of the original film S. A photosensitive material such as a PS plate P is placed below the transparent plate 105 and under the original film S. Light is emitted from a light source provided above the transparent plate 105 in this state. Consequently, an image of the original film S is formed on the photosensitive surface such as the PS plate P in contact with the original film S through the mask opening M (defined by M.sub.X and M.sub.Y) formed on the transparent plate 105. After the image of the original film S is formed at a prescribed position, the masking sheets 112a and 112b are arranged such that a different unexposed area only is opened. Thereafter, the image of the original film S is formed on the photosensitive material through the same operations. By repeating the same process, a plurality of the images of the original film S are formed on the photosensitive surface P.
As described above, in a composer or the like, a plurality of originals are formed on the transparent surface P with end surfaces thereof common to each other. Therefore, it is necessary to prevent blurring due to diffraction of the light at end surface portions of each of the originals.
However, since the transparent plate 105 has some thickness, the dimension of the mask opening (M.sub.X and M.sub.Y) and the dimension of an area to be exposed (E.sub.X, E.sub.Y) do not coincide with each other. Consequently, blurring due to the diffraction of the light from the light source inevitably occurs around the area to be exposed E.sub.X, E.sub.Y on the photosensitive surface (see FIGS. 11A and 11B).
In the above described conventional device, the leading edges 113a and 113b are formed of thick plate material (about 3 mm). When a pair of leading edges 113a are arranged intersecting with and over the other pair of leading edges 113b arranged close to the transparent plate 105, the distance between the masking sheet 112a and the original film S is the sum of the thickness of the transparent plate 115, the thickness of the masking sheet 112b (about 0.2 mm) and the thickness (about 3 mm) of the leading edge 113b. Consequently, images are blurred at broader areas around the area E.sub.X to be exposed defined by the leading edge 113a.